The present invention relates to a misfiring detecting apparatus and method for an internal combustion engine which can detect misfiring of a cylinder of the engine based on the pressure therein.
In general, internal combustion engines such as an automotive gasoline engine have a plurality (for example four) of cylinders which are operated through four cycles including an intake stroke, a compression stroke, a power stroke and an exhaust stroke. In order to properly control the ignition timings of the cylinders, the order of fuel injection into the cylinders, etc., an engine control unit in the form of a microcomputer is employed for performing various electronic calculations. To this end, based on a cylinder reference position signal representative of the crank positions of the cylinders and a cylinder identification signal which are generated by a signal generator in synchronism with the rotation of the engine, the microcomputer identifies the operating positions of the cylinders and properly controls their operations.
For example, for control of the cylinder ignition, a fuel/air mixture in each cylinder compressed by a piston must be fired for combustion at an optimum timing by a spark generated by a spark plug. In this connection, however, there are times when the mixture in a cylinder, though ignited by a spark plug, does not properly combust depending upon the state of combustion, the condition of the spark plugs, etc. In this situation, an abnormal load is applied to the remaining cylinders, giving rise to a fear of engine damage. Thus, in order to maintain safe engine operation, there is a need to detect, for each ignition cycle of each cylinder, whether the mixture in a cylinder has combusted without fail. For such a purpose, a misfiring detecting apparatus has been proposed which can determine the condition of combustion or misfiring for each cylinder by detecting the pressure therein during the power stroke thereof after an ignition thereof.
FIG. 11 illustrates an internal combustion engine with a typical example of such a known (i.e., conventional but not prior art) misfiring detecting apparatus. In this figure, the engine has a plurality of cylinders 1, only one of which is illustrated for the sake of simplification in description. The cylinder 1 includes a combustion chamber 2 defined therein. A spark plug 3 is mounted on the cylinder head with its electrodes present in the combustion chamber 2. A piston 4 is slidably received in the interior of the cylinder 1 for reciprocating motion therein due to the combustion of an air/fuel mixture in the combustion chamber 2, An intake pipe 5 is connected with the cylinder head for supplying a mixture into the combustion chamber 2 through an intake valve 7, An exhaust pipe 6 is also connected with the cylinder head for discharging exhaust gases from the combustion chamber 2 through an exhaust valve 8.
A fuel injector 9 connected to an unillustrated fuel supply is mounted on the intake pipe 5 for injecting into the intake pipe 5 an amount of fuel which is determined in accordance with the amount of air sucked into the combustion chamber 2 through the intake pipe 7 and an unillustrated throttle valve.
An orifice 2a is formed through the cylinder wall surrounding the combustion chamber 2 for introducing gases in the combustion chamber 2 into a pressure sensor 10 which is mounted on the cylinder wall adjacent the orifice 2 for sensing the pressure in the combustion chamber 2. An ignition coil 11 is connected to a power supply 12 and has a primary winding connected to an igniter 13 and a secondary winding connected to the spark plug 3.
An engine control unit (ECU) 14 in the form of a microcomputer controls the intake valve 7, the exhaust valve 8, the fuel injector 9, the igniter 13, etc. The ECU 14 also operates to detect misfiring in each cylinder 1, and to this end, it receives a crank angle signal from an unillustrated crank angle sensor representative of prescribed crank positions of a piston 4 in each cylinder 1, various signals representative of a variety of aspects of the engine operation, and output signals of the pressure sensors 10 for the respective cylinders 1 representative of the pressures P therein. Though not illustrated, the ECU 14 includes a threshold setter for setting a threshold for determining misfiring in each cylinder 1, and various kinds of calculators for performing a variety of operational calculations. For example, the unillustrated crank angle sensor senses a prescribed crank position of each piston 4 at which there will be a great or significant difference in the pressure in each combustion chamber 2 between the time when combustion takes place and the time when there is misfiring or no combustion. Such a crank position can be set to any arbitrary point between about 10 degrees after top dead center (TDC) and about 90 degrees after TDC.
The operation of the above-described conventional misfiring detecting apparatus will now be described in detail below. As referred to above, four cycles including an intake stroke, a compression stroke, a power stroke and an exhaust stroke are carried out within two reciprocating motions of a piston 4 in each cylinder 1. The ECU 14 controls the amount of fuel injected into the intake pipe 5 by the fuel injector 9 on the intake stroke, the ignition timing of each spark plug 3 and the like in accordance with the operating condition of the engine.
For example, when the intake valve 7 is opened to feed an air/fuel mixture into the combustion chamber 2, the ECU 14 controls the amount of air taken into the intake pipe 5 through the unillustrated throttle valve as well as the amount of fuel injected into the intake pipe 5 by the fuel injector 9 in accordance with the degree of opening of the throttle valve. In addition, the ECU 14 operates to drive the igniter 13 at a prescribed timing after the compression stroke so as to supply a current from the power supply 12 to the ignition coil 11 and cut off the current supply at an appropriate timing. As a result, the spark plug 3 discharges to generate a spark at its electrodes to fire the air/fuel mixture in the combustion chamber 2. Normally, such ignition timing is set to a point near a crank angle of zero degrees or top dead center.
When explosion or combustion of the mixture is caused due to the discharge of the spark plug 3, the pressure P in the combustion chamber 2 as sensed by the pressure sensor 10 sharply rises, but if no explosion or combustion takes place because of misfiring, the pressure P in the combustion chamber 2 remains unchanged or at a low value.
In view of this, the ECU 14 takes in the cylinder pressure P in the combustion chamber 2 as sensed by the pressure sensor 10 at a prescribed timing during each power stroke, generates a misfiring determining threshold, and compares the cylinder pressure P with the threshold. If the cylinder pressure P is less than the threshold, it is determined that there is misfiring in the cylinder, and a misfiring flag for the misfiring cylinder is set up.
In this case, however, if the determination of misfiring is made based solely on the result of such a comparison between the cylinder pressure P and the threshold, there is a probability that the cylinder pressure P sometimes exceeds the threshold due, for example, to an offset component which is generally contained in the output signal from the pressure sensor 10. Accordingly, even in the event that no combustion in effect takes place, it can be determined that there is normal combustion. This may result in engine damage as described above.
Thus, in the above-described conventional misfiring detecting apparatus and method, the state of combustion is determined based on whether or not the cylinder pressure P exceeds the threshold. In this case, however, if the output of the pressure sensor 10 contains, due for example to variations in manufacture, an offset component which is greater than the threshold and which is superposed on a pressure component thereof representative of the actual pressure P in the combustion chamber 2, it is determined that normal combustion takes place in the combustion chamber 2, despite the fact that there is actually misfiring therein. Therefore, in this case, it becomes difficult to detect misfiring with high reliability.